1. Field of Invention
This invention relates to the production of fiber-reinforced thermoset polymer articles that can include variations in cross section with respect to both their transverse and longitudinal axes. More specifically, it relates to a process and apparatus for producing pultruded articles that may, or may not, have variations with respect to the longitudinal axis, or machine direction, the transverse axis, perpendicular to the longitudinal axis, or both axes. In addition, use of the method and apparatus provides the capability of producing articles at a rate that improves upon the throughput of the conventional pultrusion process.
2. Discussion of Prior Art
The pultrusion process has been used extensively to mass produce fiber reinforced polymer (FRP) articles. These articles are rigid composite materials that are straight in the longitudinal direction and have a uniform cross section. The side rails of a ladder, for example, have these characteristics and are produced through a pultrusion process today. In this process, a bundle of fibers is pulled through a resin bath and into a heated die. The shape of the die opening determines the cross section of the pultruded article. The heat of the die cures the resin, solidifying the composite material. When a desired length of the solidified material has been pulled through the die, a cut-off saw cuts the material to form the pultruded article. The standard pultrusion process is capable of producing only articles that are longitudinally straight and have a uniform cross section.
A variation on the pultrusion process, described in U.S. Pat. No. 5,556,496 to Sumerak, describes a two step process wherein a specially controlled pultrusion die, described in U.S. Pat. No. 5,716,478, also to Sumerak, is used. In this process, a length of cured material is pulled through the die, followed by a length of material that is not cured because the temperature of the die has been lowered below the curing temperature of the resin. The uncured material is then reshaped and then cured by pulling it through a die with the profile of the second desired shape. Using this process, variations in the cross-section of the pultruded article can be created to introduce, for example, flanges, off-sets, bosses and the like. The disadvantages of this process are that the thermal cycling of the apparatus reduces throughput and wastes energy. In addition, after the first segment has been cured, an area of partially cured resin will extend into the uncured material, creating the possibility that final cure will not be uniform and the area of transition will be weakened.
U.S. Pat. No. 6,679,695 B2 to Kusy et al. describes a pultrusion apparatus and method for continuous fabrication of fiber-reinforced plastic articles having a non-linear shape. The method is a three step process, comprised of partially curing the material, using a mold to shape the partially cured material, and then completing the curing of the material. Using this method, the cross section of the article cannot be varied. Further, the apparatus is designed to produce only one shape, such as arch wires for orthodontic applications, as described in the preferred embodiment.
Another variation on the pultrusion process, called the, “double die” process was used by Innovative Composites Inc. to produce reinforcing bar (rebar) for reinforcing concrete. The double die process provides a single step process for producing longitudinally straight articles with a varying cross section by the introduction of a moving die that surrounds the resin impregnated fiber and is drawn into the heated static die with it. The moving die has an external cross section that conforms to the shape of the opening of the static die and an internal cross section that varies, but maintains a constant volume that corresponds to the volume of the resin impregnated fiber. The varying shape of the internal cross section of the moving die cavity determines the cross sectional shape of the article. Heat transferred from the static die through the moving die cures the resin. Although the cross section of articles produced by the double die process and the process of U.S. Pat. No. 5,556,496 may vary, the longitudinal axis of these articles must remain straight.
There is a long standing requirement for a process that can produce composite material articles with shapes that can incorporate variations in both the transverse and longitudinal directions. Currently, such articles can be produced only through time consuming manual processes, multiple step procedures, or by connecting component pieces to approximate the desired shape. This increases the cost of the article and/or decreases its strength and reliability.
The present invention provides an apparatus and an efficient method for production of FRP thermoset materials that may include variations in shape in both the transverse and longitudinal directions. In addition, it provides a method for production of FRP materials that improves the quality of pultruded thermoset articles with variations in cross section, and the throughput of the pultrusion process through a single-step, uniform curing process.
Further objects and advantages will be apparent from a consideration of the drawings and ensuing description.